The salinity of water is determined today essentially by a measurement of its electrical conductivity, at a known temperature and pressure. Conductivity sensors are however unreliable since they are very sensitive to degradation relating to the marine environment (as explained by the document “Capteurs de conductivité en océanographie: état de l'art et perspectives.” (“Conductivity sensors in oceanography: state of the art and prospects”) by M Le Menn, published in RSTD Vol. 64, pp 107-111, June 2004).
The use of other methods based on the measurement of the refractive index of sea water (as illustrated by the document “Remote refractive index difference meter for salinity sensor” by H Minato et al, published in IEEE Trans on Instrumentation and Measurements, Vol. 38, No 2, pp 608-612, 1989) or the use of fibre sensors can be envisaged.
Measurement of the index makes it possible, knowing the temperature and pressure, to find the salinity level either through the use of the equations of R. C. Millard and G. Seaver (R. C. Millard and G. Seaver, “An index of refraction algorithm for sea water over temperature, pressure, salinity, density and wavelength”, Deep-Sea Research, Vol. 37, No 12, pp 1909-1926, 1990), the accuracy of which is 0.024 ppt in salinity, or by direct calibration for temperature, pressure and salinity. Which assumes that it is possible to measure the refractive index of sea water with a resolution of at least 10−6 in oceanographical ranges.
Having regard to the intended application, the salinity sensor must be compact and of low cost. The major problem is controlling the drift in measurement with respect to the variations in temperature and pressure that the device must undergo when it is used in situ. This point is all the more critical since the device is disposable, which confirms relative independence of the sensor and low cost. During uses for long periods in a marine environment, another problem has to be resolved: that of resistance to dirt or organic deposits of all kinds or fouling, which in general terms cause degradation of the measuring devices.